A reactor is charged with (7R)-4-methoxybenzyl-3-((E)-((R)-1′-(tert-butoxycarbonyl)-2-oxo-[1,3′-bipyrrolidin]-3-ylidene)methyl)-7-((Z)-(2-hydroxybenzylidene)amino)-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate (1 eq), water (6-9 weight % relative to the Schiff base), (Z)-2-(5-amino-1,2,4-thiadiazol-3-yl)-2-(((4-methoxybenzyl)oxy)imino)acetic methanesulfonic anhydride (2.5 eq) and dichloromethane (1.75 mL per gram of Schiff base). The mixture is stirred for three hours at room temperature, then added to methyl tertiary butyl ether over the course of 2-3 hours, resulting in the formation of a solid. The solid is filtered and the filter cake was washed with additional methyl tertiary butyl ether. The filtered solid is deprotected using conventional methods and dried under vacuum to yield the title compound.
